The present invention relates to a process for the continuous thermal cracking of hydrocarbon oils.
The atmospheric distillation of crude mineral oils for the preparation of light hydrocarbon oil distillates, such as gasoline, kerosene and gas-oil, yields an asphaltenes-containing residue as a byproduct. Originally such residues used to be utilized as heavy fuel oil for low speed engines and power stations. In view of the growing demand for light hydrocarbon oil distillates and the shrinking demand for heavy fuel oils and asphalt, various treatments aiming at the preparation of light hydrocarbon oil distillates from atmospheric residues have been proposed and are commercially applied.
A well known treatment of residual oils for preparing light products is thermal cracking. For the thermal cracking of residual feedstocks two types of processes are available, namely furnace cracking and soaker cracking. Furnace cracking implies that the actual cracking takes place at the downstream end of a furnace and to some extent in the transfer line between the furnace and a subsequent treating unit. The residence time of the feedstock in the cracking zone is relatively short, of the order of only one minute. In the case of soaker cracking, the feed is heated to a suitable temperature, which is considerably lower than the temperature applied in furnace cracking, and the feed is allowed to stay at that temperature for a period of usually 10-30 minutes in a vessel known as a soaker. A soaker can be defined as an elongated vessel without supplementary heating, which vessel allows cracking to take place over a prolonged period. No heat is provided to the soaker and, since the cracking reaction is endothermic, the temperature of the oil drops by about 10.degree.-30.degree. C. during the passage through the soaker.
Soaker cracking, also known as visbreaking, has staged a come-back as a convenient and relatively inexpensive step toward reducing fuel oil residues. Especially during the last decennia, savings of production costs have become of paramount concern. The process of visbreaking has major advantages over furnace cracking, viz., lower capital costs, lower fuel consumption and longer onstream times.
U.S. Pat. No. 1,899,889 mentions a method for the thermal cracking of petroleum oils, which method comprises heating the oil, introducing the hot feed into a soaking vessel in which most of the cracking takes place and subsequently conducting the cracked liquid and formed vapors into a fractionating zone. According to this publication the hot feed is introduced and vaporous products leave through a common line at an upper portion of the vessel.
The conversion obtained by thermal cracking operations is the result of the two main operating variables, viz. temperature and residence time. The desirable effect of thermal cracking, i.e. the decrease of the viscosity of the feedstock, arises from the fact that larger molecules have a higher cracking rate than smaller molecules. At lower temperatures the difference in cracking rates between larger and smaller molecules increases and hence influences the desirable effect positively. At very low temperatures the cracking rate however decreases to uneconomically small values. In view of these aspects, the temperature in a soaking vessel is preferably chosen in the range between about 400.degree. and 500.degree. C.
The residence time in a soaking vessel depends upon the configuration and size of the vessel as well as the pressure in the vessel. High pressure will cause only a small vapor flow to be produced which results in a relatively low vapor holdup in the vessel and therefore a relatively long residence time of liquid feed. Low pressures have on the contrary a decreasing effect on the residence time of the liquid feed. At a given configuration and size of a soaking vessel, the prevailing pressure should be so chosen as to allow for a sufficient residence time of the liquid feed. The pressure is preferably in the range of from about 2 to 30 bar.
The rate of conversion, or in other words the cracking severity, is in general limited by the storage stability of the cracked product. The stability properties of the product deteriorate as the cracking proceeds. The average rate of conversion can be regulated by controlling the temperature of the feedstock and the residence time of the feed in the used soaking vessel. In soaker cracking operation a further effect, besides temperature and residence time, influences the product stability. This further effect is induced by gas formation during the cracking. Formed gas will induce back-mixing or swirl of the feedstock in the soaking vessel, causing a spread in the liquid residence time at cracking temperature. As a result thereof part of the feed gets overcracked and influences the stability of the total product from the vessel negatively, while another part of the feed gets undercracked, in that it is insufficiently converted into lighter products.
An important reduction of back-mixing in a soaking vessel may be obtained by providing the vessel with internals dividing the interior of the vessel into a plurality of compartments. Heated feed is allowed to crack in a soaking vessel in which internals, preferably formed by perforated plates, have been arranged. The swirling motion occurring in soaking vessels not provided with internals upon gas formation is in fact transformed due to the presence of such internals, into a plurality of relatively small swirls, resulting in a steep decrease of overall back-mixing and therefore an improved product stability. By increasing the number of compartments in a soaking vessel, back-mixing can be further restricted. The height of the compartments or in other words the distance between adjacent internals should however be sufficient in order to allow inspection and maintenance.
Vapors evolved in the compartments of the soaking vessel pass with the liquid feedstock/product through the upstream compartments and are recovered from the vessel together with the liquid product. If the amount of gas generated in the soaking vessel is rather moderate, the provision of compartments in the vessel will normally be sufficient for generating a product having an acceptable stability. If, however, the operating conditions and/or the composition of the feedstock are such that large quantities of gas are generated, or are already present in the feed to the soaking vessel, the compartmented division of the vessel may be insufficient for preparing products with optimal stability.
The object of the present invention is to further improve the above known process using a compartmented vessel, in order to remove gaseous products as quickly as possible so as to reduce or even prevent overcracking and thus optimizing the stability of the product prepared.